The present invention relates to drag-reducing polymer suspensions and their method of manufacture. More specifically, this invention relates to a method for preparing an ultra-high molecular weight, substantially non-crystalline hydrocarbon soluble polymer suspension.
A drag-reducing agent is one that substantially reduces the friction loss that results from the turbulent flow of a fluid. Where fluids are transported over long distances, such as in oil and other hydrocarbon liquid pipelines, these friction losses result in inefficiencies that increase equipment and operations costs. Ultra-high molecular weight polymers are known to function well as drag-reducing agents, particularly in hydrocarbon liquids. In general, drag reduction depends in part upon the molecular weight of the polymer additive and its ability to dissolve in the hydrocarbon under turbulent flow. Effective drag-reducing polymers typically have molecular weights in excess of five million.
Drag-reducing polymers are known in the art. Representative, but non-exhaustive, samples of such art are: U.S. Pat. No. 3,692,675, which teaches a method for reducing friction loss or drag for pumpable fluids through pipelines by adding a minor amount of a high molecular weight, non-crystalline polymer; and U.S. Pat. No. 3,884,252, which teaches the use of polymer crumb as a drag-reducing material. These materials are extremely viscoelastic and, in general, have no known use other than as drag-reducing materials. However, the very properties that make these materials effective as drag-reducing additives make them difficult to handle because they have a severe tendency to cold flow and reagglomerate even at subambient temperatures. Under conditions of pressure, such as stacking or palleting, cold flow is even more intense and reagglomeration occurs very quickly.
The general propensity of non-crosslinked elastomeric polymers (elastomers) to cold flow and agglomerate is well-known. Polymers of this sort cannot be pelletized or put into discrete form and then stored for any reasonable period of time without the materials flowing together to form large agglomerates. Because of such difficulties, elastomers are normally shipped and used as bales. However, such bales must be handled on expensive equipment and cannot be pre-blended. In addition, polymers such as the drag-reducing additives described are not susceptible to such balings, since cold flow is extremely severe. Further, dissolution time for such drag-reducing materials from a polymer state in the flowing hydrocarbons to a dissolved state is so lengthy as to severely reduce the effectiveness of this material as a drag-reducing substance.
Numerous attempts have been made to overcome the disadvantages inherent in cold-flowing polymers. Representative, but non-exhaustive, of such art is that described in U.S. Pat. No. 3,791,913, wherein elastomeric pellets are surface cured, i.e., vulcanized to a minor depth in order to maintain the unvulcanized interior of the polymer in a xe2x80x9csackxe2x80x9d of cured material, and U.S. Pat. No. 4,147,677, describing a method of preparing a free-flowing, finely divided powder of neutralized sulfonated elastomer by admixing with fillers and oils. This reference does not teach a method for making free-flowing powders of non-elastomeric material. U.S. Pat. No. 3,736,288 teaches solutions of drag-reducing polymers in inert, normally liquid vehicles for addition to liquids flowing in conduits. A xe2x80x9cstaggered dissolutionxe2x80x9d effect is provided by varying the size of the polymer particles. Suspension or surface-active agents can also be used. While directed to ethylene oxide polymers, the method is useful for hydrocarbon-soluble polymers as well. U.S. Pat. No. 4,088,622 describes a method of making an improved, molded drag-reducing coating by incorporating antioxidants, lubricants, and plasticizers and wetting agents in the form of a coating which is bonded directly onto the surface of materials passing through a liquid medium. U.S. Pat. No. 4,340,076 teaches a process for dissolving ultra-high molecular weight hydrocarbon polymer and liquid hydrocarbons by chilling to cryogenic temperatures comminuting the polymer formed into discrete particles and contacting these materials at near cryogenic temperatures with the liquid hydrocarbons to more rapidly dissolve the polymer. U.S. Pat. No. 4,341,078 immobilizes toxic liquids within a container by injecting a slurry of cryogenically ground polymer particles while still at cryogenic temperatures into the toxic liquid. U.S. Pat. No. 4,420,440 teaches a method for collecting spilled hydrocarbons by dissolving sufficient polymer to form a nonflowing material of semisolid consistency by contacting said hydrocarbons with a slurry of cryogenically comminuted ground polymer particles while still at cryogenic temperatures.
Some current drag-reduction systems inject a drag-reducing polymer solution containing a high percentage of dissolved, ultra-high molecular weight polymer into conduits containing the hydrocarbon. The drag-reducing polymer solution is normally extremely thick and difficult to handle at low temperatures. Depending upon the temperature of the hydrocarbon and the concentration at which the drag-reducing polymer solution is injected, significant time elapses before dissolution and resulting drag reduction. Solid polymers of these types can take days to dissolve in some cases, even though drag reduction is greatly enhanced once dissolution has finally occurred. Also, such ultra-high molecular weight polymer solutions become very viscous as polymer content increases, in some cases limiting the practical application of these solutions to those containing no more than about 15 weight percent polymer. This makes complex equipment necessary for storing, dissolving, pumping, and injecting metered quantities of drag-reducing material into flowing hydrocarbons.
Another way to introduce ultra-high molecular weight polymers into the flowing hydrocarbon stream is through a suspension. The ultra-high molecular weight polymers are suspended in a liquid that will not dissolve or will only partially dissolve the ultra-high molecular weight polymer. This suspension is then introduced into the flowing hydrocarbon stream. The tendency of the ultra-high molecular weight polymers to reagglomerate makes manufacture of these suspensions difficult. A way of controlling the tendency of the ultra-high weight polymers to reagglomerate is to partially surround the polymer particles with a partitioning agent, occasionally termed a coating material, to reduce the ability of these polymers to reagglomerate. U.S. Pat. No. 4,584,244, which is hereby incorporated by reference, describes a process whereby the polymer is ground and then coated with alumina to form a free-flowing powder. Some processes using a partitioning agent require that the partitioning agent completely surround the polymer core, which requires that at least 20% and often as much as 50% of the weight of the final composition be the coating agent. Other examples of partitioning agents used in the art include talc, tri-calcium phosphate, magnesium stearate, silica, polyanhydride polymers, sterically hindered alkyl phenol antioxidants, and graphite. Partitioning agents, however, add weight to the drag-reducing agent material, resulting in higher transport costs and additional handling equipment, without any drag-reducing benefit. Further, some partitioning agents are incompatible with the hydrocarbon fluid or may be an unwanted contaminant of the hydrocarbon fluid.
Accordingly, a drag-reducing suspension and a method of producing a drag-reducing suspension are disclosed herein. One embodiment of the present invention is drawn to a method for the preparation of a drag-reducing polymer suspension wherein an ultra-high molecular weight polymer is mixed with a grinding aid to form a polymer/grinding aid mixture. This mixture is then ground at a temperature below the glass transition temperature of the ultra-high molecular weight polymer to form ground polymer/grinding aid particles. The ground polymer/grinding aid particles are then mixed with a suspending liquid to form the drag-reducing polymer suspension. In another embodiment of the present invention, drag-reducing polymer suspension is prepared by cooling an ultra-high molecular weight polymer with nitrogen, helium, argon, or dry ice. The ultra-high molecular weight polymer is a linear poly(xcex1-olefin) comprised of monomers with carbon chain lengths of between 4 and 20 carbons. The ultra-high molecular weight polymer is mixed with a grinding aid to form a polymer/grinding aid mixture. This mixture is then ground at a temperature below the glass transition temperature of the ultra-high molecular weight polymer. The mixture is then mixed with a suspending fluid. At least one of the following components is then added to the suspending fluid: wetting agent, antifoaming agent, and thickening agent.
One advantage of the present invention is that the drag-reducing polymer suspension is easily transportable and does not require pressurized or special equipment for storage, transport, or injection. Another advantage is that the drag-reducing polymer is quickly dissolved in the flowing hydrocarbon stream. Yet another advantage of the present invention is that the extra bulk and cost associated with the inert coating agent may be eliminated, allowing easier transport. Still another advantage of the present invention is that reagglomeration of the drag-reducing polymers is greatly reduced, allowing for easier handling during manufacture. Another advantage of the present invention is that the drag-reducing polymer suspension is stable, allowing a longer shelf life and balancing of customer demand with manufacturing time. A further advantage of the present invention is that the amount of inert ingredients in the final product is reduced. In addition, manufacturing throughput is increased by the use of the grinding aid.